Heat treating process for martensitic transformation alloys



United States Patent O M 3 223,562 HEAT TREATING PROCESS FORMAR'I'ENSTIC TRANSFORMATEON ALLOYS William I. Bassett lll, Gardena,Calif., assigner, by mesne assignments, to Union Carbide Corporation, acorporation of New York Filed May 1, 1961, Ser. No. 106,865 16 Claims.(Cl. 148-125) This application is fa continuation in part of myapplications S. N. 842,427, filed September 25, 1959, now abandoned, andS. N. 65,858, led October 28, 1960, for Method and Apparatus for HeatTreating Metals.

This invention relates to the heat treatment of alloys having anaustenitic to martensitic type transformation, and has as its basicobject to improve the tensile and uniformity characteristics of Suchalloys in relation to the plasticity thereof. The invention isespecially applicable to the conditioning of ferrous all-cys having theausteniticmartensitic type phase change. The term plasticity is used ina broad sense to include all of the desirable properties of ductility`and formability, such as the ability to withstand bending anddeformation as in forming operations; machinability, i.e. theadaptability to cutting and abrading in shaping and finishingoperations, and in particular the ability to undergo the removal ofmetal smoothly with a minimum of tearing action; and malleability, Le.,the ability to undergo a high degree of deformation, as in coiningoperations, various impact extrusion operations and miscellaneousforming operations wherein considerable proportions of the alloy aremoved from one area to another, without being fractured or weakened inthe process.

More specifically, the invention is directed to the heat treatment ofaustenitic-martensitic transforming type alloys as contrasted toprecipitation-hardening alloys not having substantialaustenitic-martensitic transformation properties.

An object is to provide an improved heat-treating process wherebyimproved tensile strength may be imparted to alloys without incurringthe extent of loss of ductility, malleability, etc. which heretofore hasbeen regarded as an inevitable consequence of processing to increasetensile properties. Conversely, the invention aims to provide aheat-treated Ialloy article having improved plasticity `as definedabove, together with improved tensile and allied properties. Stated morespecifically, the invention provides for attaining such plasticproperties as are normally attained only by a considerable sacrifice intensile properties, while simultaneously attaining the high tensileproperties which can be attained, in conventional hardening processes,only through an extreme sacrifice in plasticity and which, under normalprocessing methods, are partially restored to the alloy by a processwherein the hardness and tensile properties are tempered in order toimprove the plasticity.

A further object is to provide an improved process in whichstress-relieving is `accomplished -at much lower temperatures than areutilized in normal tempering or stressrelieving operations.

Other objects and advantages will become apparent in the ensuingspecifications and appended drawing, in which:

FIG. 1 is la chart of my process, illustrating in particu- 3,223,562Patented Dec. 14, 1965 ICC lar the change in temperature level betweenthe several stages of the process; and

FIG. 2 is a chart of my process, compared to processes of the prior art.

GENERAL DESCRIPTION OF THE PROCESS The basic process, in its optimumembodiment, includes the following steps:

(l) Austenitizing, i.e. heating the alloy to a temperature .above theupper transformation temperatures Iand maintaining it at thattemperature until transformation to austenite is complete.

(2) Primary quench through the transformation range.

(3) Secondary or cool quench to approximately room temperature.

(4) Refrigerative quench.

(5) Restoration to room temperature.

(6) Stress relief.

Terminology of the arzt-In considering the differences between myprocess and known processes, it is helpful to review certain terminologyutilized in discussing austenitic-martensitic transformations. The termaustenitizing may be utilized to designate the initial heating of thealloy to `a point within a temperature range wherein its structurebecomes austenite, namely; a solid solution of carbon in iron. Therecognized temperature range for austenitizing most alloy steels extendsfrom about 1380 F. to about 1800 F.

In quench-hardening an austenitized alloy, in the higher portion of thequenching range, if the quenching rate or cooling proceeds sutiicientlyslowly, there will be formed one of the decomposition products includingferrite and cementite, which most hardening processes `attempt to avoidby quenching rapidly down to a lower temperature zone, where begins thetransformation from .austenite to the desired hard martensiticstructure. This transformation takes place through a range of decreasingtemperatures down to a completion level referred to in the followingparagraph, which range is referred to herein as the critical martensitictransformation range, or more briefly, as the transformation range.

The term Ms is commonly used to designate the upper temperature level ofythis critical transformation range, for the alloy being worked. Theterm Mf is used to designate what is commonly regarded as the lowerlevel of this range. Actually, this is a level where approximately 90%transformation to martensite has taken place, and this level is now morecommonly referred to as the M90 level rather than the Mf level. This isthe meaning applied to these terms as used in these specifications andclaims. It is generally recognized that some residual transformation ofaustenite to martensite takes place below the Mf level.

It is common practice to depict the transformation characteristics of analloy by fa chart (such as is shown in the drawing) wherein temperaturelevels are charted on a vertical Y axis and elapsed quenching time ischarted (usually logarithmically) on a horizontal X axis. In plottingthe formation of decomposition and transformation products, thereresults an area (such as the area designated A-l-F-I-C in the drawing)lying to the right of an S-curve line (TTT curve, or time, temperature,transformation curve) which area is herein referred to generally as thedecomposition products area. The invention, in common with mosthardening processes, attempts to ravoid all of the decompositionstructures in this area. Intermediate the upper and lower limits of thetemperature range from austenitic down to the Ms beginning of martensitetransformation range, the TTT curve at the margin of the decompositionproducts :area :has a bulge toward the Y axis which bulge is commonlyreferred to as the nose of the TTT curve (indicated at n in thedrawing).

My process is distinguished from pri-or known heattreating processes,principally in departing from accepted relationships between the varioussteps of austenitizing, quenching, tempering and stress-relieving stepswhich are utilized in most heat treating processes. In this respect, itdiffers from the better known heat treating processes such asMartempering and Austempering, which are currently regarded asimprovements over processes such as the well known oil-quench whereintemperature is dropped on a straight line curve through the martensitictransformation range.

The prior urn-In order to compare my process with the better knownconventional processes, the latter are here reviewed briefly.

Ol-quench.-In the conventional `oil quench, which is a relatively quickquench, the temperature is dropped rapidly on a substantially straightline curve, avoiding the nose of the decomposition products area, downto arnbient temperature (approximately room temperature) beginning at aconventional austenitizing temperature and dropping entirely through themartensite-transformation range without waiting for internal andexternal temperatures to equalize.

Martemperng-ln this more recently developed process, the alloy is rstaustenitized at a conventional austenitizing temperature, is thenquenched rapidly on a quench curve avoiding the nose of thedecomposition products area, to a temperature somewhat above theconventional Ms level, is held at this level until external and internaltemperatures have equalize/d, and is then quenched more slowly (e.g. inair) through the martensite transformation range down to roomtemperature.

Austempering.-In this process, based upon isothermal transformation ofaustenite to a non-martensitic structure, after an initial heating stepin which the alloy is austenitized at a conventional aus-tenitizingtemperature, the alloy is cooled to a temperature level somewhat abovethe Ms level, and is held at this level for an extended period of timegoing all the way through the decomposition products area above themartensite formation range (with a resultant formation of Bainite), fora sufficient time to complete such transformation. Then the alloy iscooled down to room temperature.

The invention-My invention is distinguished over all such prior heattreating processes in combination of departures ,from thecharacteristics utilized in the several steps of the known processes.More specifically, my process utilizes, in some of 4the basic heattreatment steps, departures in temperature level-s at certain stages ofthe process, time (duration of various aspects of transformation) andrate (of change between various stages and levels of transformation).For example, -the invention in general permits, if desired, the use of ahigher austenitizing temperature, for any selected alloy, than thatconventionally used for such alloy.

At vthe outset, consideration may be given to the well known fact thatduring a quenching operation, if carried beyond a maximum permissibletime interval, there arrives a point where separation between carbon andiron occurs, with a resultant softening effect (separation offerritic-pearlitic complex). Accordingly, time is a most importantfactor in a quenching operation, and in order to attain satisfactoryresults the invention utilizes a quenching step wherein the alloy isquenched at the most rapid practicalrate for the alloy, untiltemperatures has dropped past the nose of the TTT curve, so that themartensitic structure may develop without diffusion of carbon. It willof course be understood that for various different alloys, thepermissible time interval will vary.

My improved heat-treating process is especially useful when itincorporates therein the magnetic quenching phase disclosed in my twoco-pending applications identiiied above and best results are obtainedby utilizing the magnetic treatment in the quenching step. However, Ihave found that significantly improved results, though of more moderatedegree, over prior heat-treating processes, are obtained in myheat-treating process without the assistance of the magnetic treatmentduring quenching, and the present invention involves the use of theheattreating process per se, apart from the magnetic treatment.

In -my improved process, after `an initial heating step in which thealloy is austenitized, it is quenched in a hot medium such as anagitated salt bath or mineral oil, without interruption through thecritical martensite transformation range without waiting for temperatureequalization in the work piece, but instead of continuing this quenchdown to room temperature, as in the standard oil quench process, it isarrested at a temperature near the M level, and the work is held at thatlevel beyond the point where equalization of internal and externaltemperatures occurs. In holding beyond the point where temperatureequalization occurs (and where it is commonly understood that expansionand release of thermal energy has occurred) some initialstress-relieving of the martensite already formed, takes place, thusavoiding such undesirable effects. vFor most alloys, I iind it best tohold the temperature of the primary quench bath at a level somewhatbelow the M90 level, although for other alloys, the temperatureequalization level may coincide with or be somewhat above the M90 level,although below the Ms level (for example, where the alloy has a highaustenitizing temperature, and elevation of the primary quenchtemperature is beneficial in minimizing damaging or undesirable effectsof quenching stresses).

Following the completion of the primary quench, the alloy is transferredto a cooler medium (such as water) and is cooled therein down to roomtemperature; is then transferred to a refrigerated medium, maintained ata very low temperature (which may be lower than F.) and is furtherquenched therein down to the temperature of that medium; and then issubjected to a prolonged stress-relieving operation at a temperaturebelow conventional stress-relieving temperature for that alloy.

DETAILED DESCRIPTION Austenz'tizng.-In the initial step (hardening oraustenitizing) performed in a furnace, the part is heated to atemperature above the upper critical temperature of austenitictransformation, to an austenitizing temperature. More specifically, thetemperature of the part is raised to a temperature between about l500 F.and 2500 F. in the heat-treating step, depending on the specificcomposition of the alloy.

Primary quench- At the end of the austenitizing step, the part istransferred to a liquid quench medium in which the temperature ismaintained at an equalization level slightly lower than the M90temperature which for most steels is in the range between 200 F. and 750F. For example, for nickel-chrome-molybdenum steels, the quench bathwill be maintained at a temperature in the range of 200 F. to 400 F. andfor chrome-molybdenum steels, in the range of 200 F. to 500 F.

While other quenching mediums having comparable characteristics ofheat-absorption rate and adaptability for maintaining an elevatedquenching temperature within the range stated above, can be used, Iprefer to use a quench comprising a fused salt, for optimum results.

The part is retained in the salt bath (either in continuous,non-,interrupted quench -or in a cyclic isothermal quench as hereinafterreferred to) for a period of time depending upon the alloy beingtreated, during which period martensitic transformation progresses tothe point where at least 90 percent transformation is effected. Thistime interval for nickel-chrome-molybdenum type 4340 alloy steel mayrange between ten minutes and twenty minutes depending upon the sectionsize of the work being quenched. For type 431 stainless steel, theduration may range between twenty minutes and one hour for comparablesection sizes.

In actual practice of the invention, I have obtained good results byutilizing a cyclic quench wherein the part is subjected to two or moreperiods of treatment in the salt bath, interrupted by a rapid quenchdown to approximately room temperature. For example, good results havebeen obtained by initially quenching the part isothermally in the saltbath for a period of about ten minutes, then dropping the temperature toroom temperature as rapidly as possible (e.g. in a water quench) thenplacing the part back in the salt bath and holding it therein for asecond period of approximately ten minutes. The time interval at roomtemperature between the two salt bath treatments is quite short, theprocedure preferably being one in which the part is placed back in thesalt bath for the second period of isothermal treatment immediately uponreaching room temperature in the water quench. Actually, the secondtreatment in the salt bath involves some stress-relieving operation torelieve quench stresses and resultant cracking or other undesirableresults thereof.

The end of the quenching operation described above completes the primarystage of quench processing for alloy compositions which undergo thedesired phase transformation at relatively high temperatures. The partis then subjected to the secondary quench.

Secondary or cool quench- In the secondary quench, which may endure fora period as short as one minute, the temperature of the parts is quicklyreduced from the temperature of the primary quench bath down to roomtemperature. The secondary quench may utilize a conventional water bathmaintained at about room temperature (eg. 70 F.). In this quench, thetransformation from austenitic to martensitic phase is substantiallycompleted, although some latent austenite (e.g. a minor percentageremaining after the completion of the martensitic transform-ation stagedescribed above) may remain.

Refrigeratve qhench.-The parts are then immediately transferred from thesecondary quench to the refrigerative quench wherein any latentaustenite is transformed to the martensitic state. In the refrigerativequench, the parts are subjected to a refrigerated medium (e.g. Dry Iceand alcohol) at an extremely low temperature, in the range of 100 F, to150 F., for an extended period of time which may extend to three hours.

Restoration to room temperatura-At the end of the refrigerative step,the parts are allowed to return to room temperature.

Stress relief.--Internal stresses induced in the metal by the treatmentreceived in the three stages of quenching, are relieved, subsequent tothe refrigerative quench by a stress-relieving operation to which thepart is subjected for a prolonged period of time which may range all theway from one hour to 120 hours or more. The temperature as applied tothis stress-relieving operation may be in the range of 200 F. to 300 F.for lowalloy steels and martensitic stainless steels considerably lowerthan the temperatures utilized in stress-relieving such alloys inconventional processes up to a range of 700 F. to 1000 F. for toolsteels including ch-rome steels.

The drawing-Referring now to the drawing, and to FIG. l specifically,the chart shown therein designates quenching time upon the X axis,reading from left to right, and designates temperature in degreesFahrenheit, on the Y axis. The TTT curve is so designated in the chart.At the upper left hand corner of the chart, the broken line t indicatesoutside temperature of a part being quenched and the broken line t'designates internal temperature. The point of intersection of the twolines t and indicated at indicates the stage in a quenching operationwhere these temperatures are equalized. For the particular alloy beingcharted, the area labeled A-l-F-i-C (for austenite plus ferrite pluscementite) designates the area of formation of such decompositionproducts in a conventional quenching operation, occurring at the maximumtime interval ending at l. In this area, the resultant structure willnot have the properties sought in the heat treating process, because ofdiffusion of carbon. The horizontal line Ms designates the temperatureat which martensitic transformation begins as cooling progresses, andthe line M00 designates the temperature level at which martensitictransformation is nearly (e.g. percent) complete. This line is commonlydesignated the Mf line. The area embraced between the Ms and M00 linesrepresents the martensitic transformation range.

The vertical lines t, t', progressing from top to bottom, indicate thecooling occurring in the quench step (eg. cooling from austenitizingtemperature to a temperature below the M00 temperature). As in mostconventional hardening processes, the cooling curve t, t' descendssufciently vertically (rapidly) to avoid the nose n of the TTT curve.The broken line w of the chart indicates a period of time during which apart is subjected to the prima-ry isothermal quenching step of myprocess, and its level indicates the temperature (near the M00 level) atwhich this step is sustained after the initial, quick temperature dropindicated at t, t. The descending line d designates the further coolingoccurring during the water quench or secondary quench. The broken line vindicates a further time interval in which the secondary quench takesplace, and its level indicates the equilibrium temperature level of thissecondary quench step.

The descending line d indicates cooling from room temperature to therange of F. or lower which occurs in the refrigerative quench and thebroken horizontal line z indicates the time interval of such step. Thelevel of this line z indicates the temperature equilibrium attained inthis step. The diagonally ascending broken line r indicates the finalstress relieving step wherein the temperature is raised to a level whichremains below the tempering level of normal processes and is held atthat level for a prolonged time period which in most cases is not lessthan 20 hours.

In the refrigerating step, the extremely low temperature to which thework is subjected results in latent transformation into substantially100 percent martensite.

In the further and nal step of stress relief, in which latent stressesare removed, I find that it is possible to attain substantially completestress relief within a considerably lower temperature range than isutilized in conventional processes, the maximum temperature to which thework is elevated in my process being in the order of 200 F. lower thanthose considered necessary in conventional processes. I attribute thisto the fact that the austenitic-martensitic transformation stressesdeveloped in my process are -only a small fraction of the extent of suchstresses developed in conventional processes.

Referring now to FIG. 2, the disclosure of my process thereincorresponds generally to that shown in FIG. l (with the exception thatit does not include the steps following the secondary quench). FIG. 2further discloses in contrast to my process, the better known prior artprocesses, namely, austempering, martempering and oil quench,represented by respective broken lines (dot dash line for austempering,dash line for martempering and dotted line for oil quench) andrespectively labeled by these names. FIG. 2 illustrates graphically how,in the austempering process, temperature equalization is performed at atemperature level above the martensite transformation range, goingthrough the decomposition products'area, with a resultant formation ofbainite. The curve for martempering is shown at e as effectingequalization at a level above the Ms level and then proceeding slowlythrough the martensite range as indicated by the diagonal portion of thecurve). The oil quench curve indicates the continuous uninterruptedquenching downwardly through the martensite transformation range anddown at q to -room temperature Without interruption.

I find that the grain structure as fixed in my process, does not undergoany noticeable further transformation even over many months ofobservation, and accordingly, I have concluded that the combination ofprocessing steps in my process results in a stable, stress-relievedcondition in the meal which remains essentially permanent.

As the final result of my process, I achieve greatly increased tensileproperties for any given degree of plasticity.

Examples of use of process-The invention may be more specicallyidentified by the following specific examples of the process as appliedto representative ferromagnetic alloys, compared to a conventionaloil-quench.

Example 1.--Type 4130 chromium molybdenum alloysteel-non-magnetiC--non-cyclc quench Analysis: (percent by weight)carbon 0.28/033, manganese, OAD/0.60; phosphorus, 0.04 max.; sulphur,0.04 max.; silicon, O/0.35; nickel, none; chromium, 0.80/1.10 molybdenum0.15/1.10.

Austenitize to l625 F.-l800 F. range.

Primary quench, non-cyclic, non-magnetic, in salt bath at 300 F.Duration, 10 min. Terminal temperature about 300 F.

Cool quench in water at room temperature, beginning immediatelyaftercompletion of primary quench. Duration, 1 min. Terminaltemperature, room temperature.

Refrigerative quench in refrigerated brine solution at about 100 F.,beginning directly after secondary quench. Duration about 3 hours.

Restoration to room temperature, following refrigerative quench.

Stress relief-reheat to 212 F. Soak at that temperature for 20 hours andair cool.

Comparative results-Example I as compared to conventional heat treatmentof type 4130 alloy steel quenched in oil from 1550 F. austenitizingtemperature-tempered by drawing at 800, on a 1" round specimen.

Convention- Processed by the ally Processed Invention 280,600 max.Tensile Strength, p.s.1 210,000 265,100 average.

251,500 min. 12.0% max. Elongation 11% 10.3% average.

7.0% min. 39.8% max. Reduction o Area 44% 33.5% average.

27.8% min.

Example Il.-Type 4340 nickel-chromiam-molybdenum alloysteel-non-magnetic, non cyclic quench Restoration to room temperature,following refrigerative quench.

Stress relief-reheat to about 212 F. and hold at that temperature forabout 20 hours. Then air cool.

Comparative results-Example II as compared to conventionalheat-treatment of type 4340 alloy steel quenched in oil from 1500 F. andtempered at 400 F.

GONVENTIONALLY PROCESSED Yield, Tensile, Elongation, Reduction of p.S..p.s.i. percent Area, percent PROCESSED BY INVENTION Test 712K 333, 000348, 000 10.0 287 297, 000 320, 000 s. o 3e. 1 340,200 10.0 27.0 M341,200 11.0 2s. 4

Example IIL-Type 4340 chromium nickel-molybdenum alloy steel-nonmagnetic, cyclic quench Analysis: Carbon 0.38/ 0.43, manganese 0.60/0.80; phosphorus 0.025 max., sulphur 0.025 max.; silicon 0.20/ 0.35nickel 1.65/ 2.00; chromium 0.70/ 0.90; molybdenum O20/0.30.

Austenitize to 1700 F.

Primary quench-Four cycles of 5 minutes each in 300 F. salt bath,following one another Without delay between cycles. Beginningimmediately after austenitizing completed. Terminal temperature about300 F.

Cool quench-in water at room temperature, beginning immediately afterprimary quench. Duration, about 1 minute.

Refrigerative quench-in refrigerated brine solution at about --112 F.,beginning directly after secondary quench. Duration about 3 hours.

Restoration to room temperature, following refrigerative quench.

Stress relief-reheat to about 212 F. and hold at that temperature forabout 18 hours, then air cool.

Comparative results-Example III as compared to conventional heattreatment of type 4340 alloy steel quenched in oil from 1500 F. andtempered at 40G-500 F.

CONVENTIONAL PRO CESS Tensile, Elongation, Reduction Yield, p.s.1.p.s.i. percent of Area, percent PROCESSED BY INVENTION Test JBX762 CodeNo. II-

NM40 340, 000 l0 27 Test No. 2 g 341, 200 11 28. 4

Example I V.-T ype 4130 chrome-moly alloy steel--nonmagnetic cyclicquench CONVENTIONALLY PRO CESSED Tensile, Elongation, Reduction Yield,p.s.. p.s.i. percent of Area, percent PROCESSED BY INVENTION Test .TBX720 274, 200 11 31 'Test N0. 2 266, 300 10 34. 3

While the examples given above are based upon data derived from testsusing the cyclical form of the primary quench operation, I find, fromcomparative test using the non-cyclic or single stage primary quench,that substantially the same results are obtained in the single stagequench.

I claim:

I. An improved process of effecting an austenitic to martensitic typetransformation in an alloy subject to such transformation, including thefollowing steps: heating the article to and maintaining it at anaustenitizing temperature until a homogeneous solid `solution ofaustenite is obtained; then rapidly quenching the article in a primaryquench through its critical martensitic transformation range in whichabout 90% of martensite is formed; arresting said quench at anequalization temperature approximately equal to the temperature at whichsaid 90% of martinsite is formed; subjecting said alloy to saidequalization temperature while external and internal temperatures of thearticle are equalized; and then further quenching the article from saidtemperature level down to room temperature.

2. The process defined in claim 1, wherein said primary quench isperformed in a liquid quenching medium maintained at a temperature nearsaid lower limit of said transform-ation range.

3. An improved process of effecting an austenitic to martensitic typetransformation in an alloy of the group including chromium molybdenumalloy steel and nickel chromium molybdenum alloy steel subject to suchtransformation, including the following steps: heating the article toand maintaining it at an austenitizing temperature until a homogeneoussolid solution of austenite is obtained; then quenching the article in aprimary quench, at a rate sufficiently rapid to avoid formation oftransformation products other than martensite, through the criticalmartensitic transformation range in which about 90% of martensite isformed; arresting said primary quench at an equalization slightly belowthe temperature at which said 90% of martensite is formed; holding thearticle of said equalization temperature slightly 'below the temperatureat which said 90% of martensite is formed; holding the article at sai-dequalization temperature until external and internal temperatures aresubstantially equalized; and then quenching in a secondary quench fromsaid temperature level down to room temperature.

4. The process defined in claim 3, wherein said primary quench isperformed in a hot liquid quenching medium maintained at saidtemperature level just below said lower limit of the criticaltransformation range; and wherein said second stage quench is performedat a slower rate than said first stage quench.

5. The process defined in claim 4, wherein said primary quench isperformed in a salt bath and said secondary quench is a water quench.

6. An improved process of effecting an austenitic to martensitic typetransformation in an alloy of the group including chromium molybdenumalloy steel and nickel chromium molybdenum alloy steel subject to suchtransformation, including the following steps: heating the article toand maintaining it at an austenitizing temperature until a homogeneoussolid solution of austenite is obtained; then quenching the article in aprimary quench through its critical martensitic transformation range inwhich about of martensite is formed without waiting for equalization ofinternal and external temperature, so as to avoid formation ofdecomposition products, to a temperature level substantially at the M-90temperature of said alloy; arresting said quench at said temperaturelevel in an equalization step until internal and external temperaturesare substantially equalized; then quenching the article in a secondaryquench from said temperature level down to room temperature.

7. The process defined in claim 6, wherein said equalization step iscontinued for a period of time extending beyond the point where thetemperature equalization is substantially completed, so as to effectinitial stress relief in martensite formed up to that point, thusavoiding undesirable effects of quenching stresses.

8. An improved process of effecting an austenitic to martensitic typetransformation in an alloy selected from the group including chromiummolybdenum alloy steel and nickel chromium molybdenum alloy steel andhaving austenitic to martensitic transformation properties comparable tothose of 4130 chrome moly steel and 4340 nickel chrome moly steel,including the following steps: heating the article to and maintaining itin an austenitizing temperature until a homogeneous solid solution ofaustenite is obtained; then quenching the article in a primary quench,rapidly through its critical martensitic transformation range in whichabout 90% of martensite is formed, down to an equalization temperaturelevel approximately at the M-90 temperature of said alloy, in a heatedliquid quenching medium maintained at said equalization level; holdingthe article in said liquid quenching medium at said equalization leveluntil external and internal temperatures of the article aresubstantially equalized; then removing the article from said liquidquenching medium and quenching it in a cooler medium down to roomternperature.

9. The process dened in claim 8, including the subsequent step ofsubjecting the article to refrigeration at a sub-zero F. temperature tostabilize its transformed structure.

10. The process defined in claim 9, including the further subsequentstep of tempering the stabilized alloy at a temperature lower than therecognized tempering level for that alloy.

11. The process defined in claim 10, wherein, in said tempering step,the alloy is held at the tempering level for a period of timeconsiderably longer than the recognized tempering period for that alloy.

12. An improved process of effecting an austenitic to martensitic typetransformation in an alloy selected from the group including chromiummolybdenum alloy steel and nickel chromium molybdenum alloy steel andhaving austenitic to martensitic transformation properties comparable tothose of 4130 chrome .moly steel and of 4340 nickel chrome moly steel,including the following steps: heating the article to and maintaining itat an austenitizing temperature until a homogeneous solid solution ofaustenite is obtained; cyclically quenching the austenitized article ina primary quench in a heated liquid quenching medium maintained at anequalizing temperature level just below the lower limit of the criticalmartensitic transformation range in which about 90% of martensite isformed for that alloy, in a series of stages including a rst stage inwhich 'i l the -article is cooled from the austenitizing range throughsaid critical transformation range to said equalizing temperature level,avoiding vformation of decomposition products and without awaitingequalization of external and internal temperatures in the article, astage of arrested cooling in which the article is held at saidequalization temperature level in said heated liquid quenching mediumfor `an extended period of time until its internal and externaltemperatures are substantially equalized, followed by a second stage ofquenching in a cooler quenching medium from said equalizing temperaturelevel down to room temperature, followed by a reheating of the articlein said heated quenching medium back to said equalizing ternperaturelevel; and then further quenching the part in a secondary quench in `acooler quenching medium from said equalizing temperature down to roomtemperature.

13. The process defined in claim 12, wherein said reheating step isperformed by holding said article in said heated liquid quenching mediumfor an extended period of time approximately las long as that of saidtemperatureequalizing step.

14. The process defined in claim 12, including the further step,following said cyclic primary quench, of subjecting the article torefrigeration at a sub-zero F. temperature to stabilize the transformedalloy structure.

15.1'1he process defined in claim 12, including the further steps,following said cyclic primary quench, of rst `subjecting the article torefrigeration at a sub-zero F. temperature to complete thetransformation; and thereafter tempering the alloy over a prolongedperiod of time at a temperature lower thanr the recognized temperingrange for that alloy.

|16. The method deiined in claim 12., wherein, in sai stage of arrestedcooling, the article is held at the equalization temperature level for aperiod of time extending beyond the point where temperature equalizationis substantially completed, so as to effect initial stress relief inmartensite formed up to that point, thus avoiding undesirable effects ofquenching stresses.

Ratei-ences Cited by the Examiner UNITED STATES PATENTS 1,924,099 8/1933Bain@ et a1. 14s-143 12,350,532 6/1944 Richardson 14s-144 2,441,6285/1948 Gritmhs et a1. 14s- 143 OTHER REFERENCES' Metals Handbook: 1948edition, published by the A.S.M., page 668, Fig. 2; and table on page307 relied upon.

Atlas of Isothermal Transformation Diagrams: by U.S. Steel Corp. page 38relied on.

A.I.M.E. Trans: 1930, Transformation `of Austenite at ConstantSubcritical Temperatures, by Davenport and Bain, page 12 relied on.

The Making, Shaping and Treating of Steel: by U.S. Steel Corp., page S11relied on.

Steel and lts Heat Treatment: vol. 1 by D. K. Bullens, page 459 reliedon.

DAVID L. RECK, Primary Examiner.

RAY K. WINDHAM, ROGER L. CAMPBELL,

Examiners.

1. AN IMPROVED PROCESS OF EFFECTING AN AUSTENITIC TO MARTENSITIC TYPETRANSFORMATION IN AN ALLOY SUBJECT TO SUCH TRANSFORMATION, INCLUDING THEFOLLOWING STEPS: HEATING THE ARTICLE TO AND MAINTAINING IT AT ANAUSTENITIZING TEMPERATURE UNTIL A HOMOGENEOUS SOLID SOLUTION OFAUSTENITE IS OBTAINED; THEN RAPIDLY QUENCHING THE ARTICLE IN A PRIMARYQUENCH THROUGH ITS CRITICAL MARTENSITIC TRANSFORMATION RANGE IN WHICHABOUT 90% OF MARTENSITE IS FORMED; ARRESTING SAID QUENCH AT ANEQUALIZATION TEMPERATURE APPROXIMATELY EQUAL TO THE TEMPERATURE AT WHICHSAID 90% OF MARTINSITE IS FORMED; SUBJECTING SAID ALLOY TO SAIDEQUALIZATION TEMPERATURE WHILE EXTERNAL AND INTERNAL TEMPERATURE OF THEARTICLE ARE EQUALIZED; AND THEN FURTHER QUENCHING THE ARTICLE FROM SAIDTEMPERATURE LEVEL DOWN TO ROOM TEMPERATURE.